The present invention relates to permanent magnet generators (PMG). In particular, the present invention relates to a system and method of regulating the output of a permanent magnet generator.
Generators are electromechanical devices that convert mechanical energy to alternating current (AC) electrical energy. Generators may receive mechanical energy from any number of sources. For instance, a generator may be provided with mechanical energy by a combustion engine such as a diesel engine. A magnetic field generated in the rotor of the generator is caused to rotate by the mechanical energy provided to the generator. The rotating magnetic field interacts with conductors wound in coils around an iron core (commonly referred to as the stator windings, resulting in AC current being generated within the stator windings. The magnitude of the AC current generated within the stator windings is directly related to the speed of the rotor and the strength of the magnetic field generated by the rotor.
Generating the magnetic field in the rotor can be done in a number of ways. For example, in wound field synchronous generators the rotor may include coils (commonly referred to as field windings) that are energized by providing current to the field windings. The AC power generated by the wound field synchronous generator is controlled by selectively varying the magnitude of the current provided to the field windings, and therefore controlling the strength of the magnetic field generated by the rotor. In the alternative, field windings wrapped around the rotor may be replaced by permanent magnets in what is known as a permanent magnet generator (PMG) or permanent magnet alternator (PMA). These types of machines are generally more efficient and robust than the wound field synchronous machines. Unlike wound field generators, the magnitude of the magnetic field generated by the permanent magnets of a permanent magnet machine is constant. Therefore, one common way of controlling the output voltage generated by a permanent magnet machine is to control the speed of the prime mover providing mechanical power to the PMG. This drawback makes PMGs impractical in a number of applications, or requires PMGs to include additional hardware such as shunt voltage regulators in order to reduce AC power generated at increased rotor speeds.